gid: default to "null"

using `|json` would let the state get an error if gid is a string, as it would
not be converted.

Using directly the `None` yaml value, and letting numbers as string be
converted by yaml parser.

users/init.sls

 users_group_present_{{ group }}:
   group.present:
     - name: {{ group }}
-    - gid: {{ setting.get('gid') }}
+    - gid: {{ setting.get('gid', "null") }}
     - system: {{ setting.get('system',"False") }}
     - members: {{ setting.get('members')|json }}
     - addusers: {{ setting.get('addusers')|json }}